Not applicable.
This invention relates generally to optical telecommunications networks, and in particular to methods and apparatus for installing fiber optic communication cables within a protective conduit such as an underground duct.
Various factors must be considered when a fiber optic cable is installed in a protective duct. A major concern is avoidance of damage to the cable during installation. Another concern is ease of installation and the desire for a reduction in the amount of time needed to install the cable. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the cable run.
Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose a tubular conduit, whose interior may have a lower coefficient of friction than the existing duct, is installed in the existing protective duct, thereby establishing a separate channel in which a cable, optionally at a later time, can be blown or flowed through the protective duct over a greater length.
U.S. Pat. Nos. 4,850,569 and 4,934,662 to Griffioen et al. describe combining high speed air flow with a pushing force applied at the entry end of the conduit to install a traditional (i.e. with non-negligible stiffness) cable. U.S. Pat. Nos. 5,197,715 and 5,474,277 to Griffioen further describe the use of a guide shuttle attached to the lead end of the cable which adds a tension force on the lead end of the cable, in addition to the motive forces applied to the cable via the high speed moving air.
During blowing/pushing installation of a cable, the propelling air-drag force developed by the volumetric flow of air through the duct is proportional to the compressor output pressure and cable diameter. However, the frictional load imposed by rubbing against the duct interior sidewall is proportional to the cable weight, hence to the square of the cable diameter. Moreover, a cable that fills the duct for a large part is subjected to extra friction caused by bends and undulations in the duct due to the stiffness of the cable, which increases with the fourth power of the cable diameter. On the other hand a cable that just fits in the protective duct can be pushed harder without buckling, but the frictional loading caused by rubbing engagement of the cable against the protective duct imposes a limit on the continuous installation length that can be obtained by pushing/blowing.
A well-known method for installing cables in ducts is a synergy of pushing and blowing as described in U.S. Pat. No. 4,850,569 to Griffioen. This method is being used now for a wide variety of cables and ducts, from small (4 mm optical cables in 7/5.5 mm guide tubes) to large (35 mm copper balanced cables in 63/50 mm ducts). The theory of this technique is described in EP 0734 109 B1 (Griffioen). According to this theory, cables with only a little play in the duct can be installed over long distances. Although the stiffness of the cable contributes more to the friction when passing bends and windings (undulations) in the trajectory, pushing becomes more efficient because the cable has less play to develop buckling.
The results of installing cables in ducts with only a little play are not predictable. Sometimes the performance is as expected but more often the blowing lengths are considerably less. Less than satisfactory performance has been experienced in many countries, e.g. in The Netherlands where such problems have occurred for many years now when installing 17 mm optical cables in 32/26 mm ducts. For this no explanation was found until now.
Typically, fiber optic cable is manufactured and then tightly coiled about a reel for ease of storage, shipment and handling. The cable reel is positioned for rotation adjacent an insertion station, and the cable is unwound from the reel and fed into an open duct. As a result of being tightly wound about the reel, residual stresses are induced into the winding coil turns of the cable. Thus the cable as it is unwound is subject to xe2x80x9ccoil-set,xe2x80x9d in which the cable develops an arcuate bend that lies in a plane which is referred to as the arc plane. As the cable is unwound from the reel, the residual stresses cause it to curve and attempt to recover to its previous coiled shape, thus producing a series of undulations or bends of a slightly helicoidal form in the arc plane. Besides the coiled shape the cable also may suffer from xe2x80x9cmini-bends.xe2x80x9d These bends are irregular and can be caused by the coiling process when not perfectly done, e.g. warping when turns cross each other. Other steps in the production and handling of the cable, e.g. bending, twisting or kinking during installation, also contribute to these mini-bends. All of these cable bends substantially limit the installation length of the cable run.
The limiting effect can be understood by realizing that cable mini-bends of only a few millimeters in amplitude can cause substantial friction between the (stiff) cable and the tubular conduit in which there is only minimum clearance, for example 1.5 mm play. The relative effect of the cable stiffness as the cable traverses winding undulations of the duct trajectory is characterized by the ratio of the normal force WB resulting from the cable stiffness with respect to the normal force of the effective cable weight Wf (normal weight in air, in liquid correction for floating). This relationship can be expressed as follows:             W      B              W      f        =            3      ⁢      AB              2      ⁢                                    W            f                    ⁡                      (                          P              /              4                        )                          4            
Here A and P are the amplitude and period, respectively, of the winding undulations in the duct trajectory and B is the cable stiffness. Amplitudes of about 5 cm in windings of ducts or guide tubes, present in many practical trajectories, usually do not contribute significantly to the stiffness friction. For protective ducts of e.g. 40/32 mm the period of the windings is usually large, e.g. 10 m. In this case the contribution of the cable stiffness B to the friction between cable and duct is negligible. For a cable with e.g. a diameter of 10 mm (free play 22 mm), a stiffness B of 1 Nm2 and a weight of 1 N/m the ratio WB/Wf is only 0.0015. In the case of a tight fitting cable with a diameter of 17 mm, a weight of 2.5 N/m and a stiffness of 5 Nm2, to be installed in a 32/26 mm duct (usually shorter period of windings, e.g. 6 m; free play 9 mm), the ratio WB/Wf becomes 0.027, still negligible.
For the smaller 7/5.5 mm guide tubes the winding period is usually less, e.g. 1 m, which would result in a much higher contribution to the friction according to the 4th power dependency of the winding period. Even with the smaller stiffness of e.g. 0.06 Nm2 of the 4 mm cable (weight 0.1 N/m) that fits (free play 1.5 mm) into the 7/5.5 mm guide tube the ratio WB/Wf with its value of 11 is much higher here. Note that this factor would mean a reduction in blowing distance by an order of magnitude. Fortunately, the stiffness of the guide tubes is so low that they can adjust to the shape of the cable easily, without causing too much friction.
For mini-bends in the stiff cables the amplitudes are much smaller and the periods shorter, e.g. 20 cm. Treating these mini-bends the same way as the windings in the tubes a ratio WB/Wf of 180 is found for a winding amplitude of only 2 mm. Again the low stiffness of the guide tubes reduces this effect, but a severe reduction in blowing length remains. This numerical example only serves as illustration. The estimation of the stiffness friction effect is not accurate, and the effect of the mini-bends in the cable has to be determined by trial and error.
Another example is the tight fitting 17 mm cable in the 32/26 mm duct. Small bends in the cable (resulting from handling or winding the cable) with only 1 cm in amplitude and 1 m in period result in a ratio WB/Wf of 4.2. In this example the duct does not adjust to the shape of the cable and installation length is substantially reduced. The cable straightening method of the present invention reduces the amplitude of the mini-bends, making possible much longer installation runs.
It has been found in field testing, for example with 4 mm optical cables encased within a steel tube (i.e. very stiff for the size) blown into 7/5.5 mm mini-tubes, that straightening the cable (by means of a straightener roller set) before entering the blowing unit will improve the installation length surprisingly by more than 30%. For cables where mini-bends are more severe (i.e. clearly visible) the improvement becomes even greater.
The method of using a cable straightener before insertion will result in improved blowing performance for any cable with non-negligible stiffness and that exhibits shape recovery xe2x80x9cmemory,xe2x80x9d such as stiff cables enclosed within steel tubes, cables enclosed within aluminum laminated polymer sheets, etc. The beneficial effect of the cable straightener is most pronounced when the relative play of the cable is less and when the stiffness of the cable is larger.
Of special interest is the use of the cable straightener in combination with the flowing installation method. Here the friction between cable and duct is minimized because of floating the cable in the liquid (typically water) used for propelling the cable. Extremely long lengths can be installed in theory when the density of the cable is chosen close to the density of the liquid used for the installation. The only contribution to the friction that remains is the stiffness of the cable in bends and windings in the duct trajectory. And, for tight fitting and stiff cables an even larger contribution to the friction (this effect is already present when installing with compressed air) is expected due to mini-bends in the cable, where the ratio WB/Wf becomes very large for small Wf. Pre-straightening the cable will have a much larger beneficial effect in that case.